1. Field of the Invention
The present invention relates to an ultrasonic sensor for radiating and/or detecting ultrasonic energy, and more particularly to an ultrasonic sensor to detect the existence of objects or to measure a distance to an object, used as, for example, an obstacle detecting sensor, a vehicle back sonar or corner sonar, etc.
2. Description of the Related Art
There has been known an ultrasonic sensor which radiates (or transmits) ultrasonic pulses intermittingly and detects (or receives) the ultrasonic pulses reflected by an object, thereby detecting the existence of the object or measuring the distance to the object. FIG. 1 shows a conventional ultrasonic sensor used in vehicles as a back sonar or a corner sonar. The ultrasonic sensor 1 comprises a metallic case member 2. The case member 2 has a hollow having a circular cross section, thereby forming a circular cylindrical shape. A flat piezoelectric element 4, accommodated in the hollow 3, is affixed to the inside surface of a bottom portion 2a of the case member 2. one of the signal wires 6 of a connector cable 5 is connected to one of the electrodes of the piezoelectric element 4, while the other signal wire 6 is in electrical conduction with the other of the electrodes of the piezoelectric element 4, through the case member 2. A soundabsorbing material 7 such as felt covers the piezoelectric element 4, and an insulating resin 8 such as silicone rubber or urethane rubber seals the piezoelectric element 4 and the sound-absorbing material 7.
Such ultrasonic sensors are mounted to, for example, an automobile bumper (not shown) to be used as back sonars or corner sonars for obstacle detection. When the ultrasonic sensor is being mounted to the bumper, the bottom portion of the case member to which the piezoelectric element 4 is affixed, is set substantially perpendicular to the surface of roads or the ground, so as to be positioned to face a direction from which ultrasonic energy is radiated. In such ultrasonic sensors, when the ultrasonic radiation range and the detection range in the horizontal direction is too narrow, a dead angle occurs in the detecting range, whereas when the ultrasonic radiation range and the detection range in the vertical direction is too wide, the reflected ultrasonics from the ground become too noisy. Therefore, in the above-described ultrasonic sensor 1, an ultrasonic horn 9 is mounted to the outer side of the case member 2 from outside the case member 2 in order to control the ultrasonic wave radiation and detection range such that the ultrasonic radiation and detection range is wide in the horizontal direction and narrow in the vertical direction.
However, when the directional properties of ultrasonic sensors are controlled by mounting an ultrasonic horn thereto, rain water and mud (splashed from the road by tires of vehicles), dust, or the like, may accumulate and get clogged in the ultrasonic horn, resulting in malfunctioning of the ultrasonic sensor. In addition, when the ultrasonic horn gets deformed, the directional properties of the ultrasonic sensor change. Further, the use of ultrasonic horns results in large ultrasonic sensors.
To overcome the above-described problems, Japanese Laid-Open Patent Publication No. 9-284896 discloses a structure for controlling the directional properties without the use of ultrasonic horns. As shown in FIGS. 2A and 2B, an ultrasonic sensor 11 has a case member 12 in which an oblong or elliptical cylindrical hollow 13 is provided. A disk-shaped piezoelectric element 15 is affixed to a bottom portion 14 of the case member 12. When the case member 12 with such a structure is used, the ultrasonic energy spreads wider in the longitudinal direction of the oblong or elliptical cross section than in the transverse direction. Therefore, the ultrasonic radiation and detection range can be made wide in the horizontal direction and narrow in the vertical direction.
For example, when the ultrasonic sensor has an outside diameter D of 18 mm, the ultrasonic wave radiation and detection range becomes 80 degrees in the horizontal direction and 60 degrees in the vertical direction, thereby exhibiting an anisotropic radiation and detection range without using an ultrasonic horn.
However, when the ultrasonic sensor with the above-described structure is made small, the ultrasonic radiation and detection range in the vertical direction becomes wide, causing the difference between the ultrasonic radiation and detection range in the horizontal direction and that in the vertical direction to be small, so that the small ultrasonic sensor is not very anisotropic.
In FIG. 2A, the vertically formed inside wall surfaces at the edges of the hollow 13 are made to extend in the longitudinal direction of the oblong such that each has an overall length H of 13 mm and a width W of 8 mm. The case member 12, in which the hollow 13 is formed, has an outside diameter D of 14 mm. The piezoelectric element 15, with a diameter d of 7 mm, is accommodated in the casing member 12, and affixed to the inside surface of the bottom portion 14. In FIG. 2B, the bottom portion 14 of the case member 12 has a uniform thickness T of 0.7 mm. The minimum thickness of each of the side walls of the case member 12 is 0.5 mm. The directional properties, in the horizontal and vertical directions, of such an ultrasonic sensor with the aforementioned dimensions are as shown in FIG. 3. FIG. 3 shows that the ultrasonic radiation and detection range (a half-decay angle) in the horizontal direction is 80 degrees, while that in the vertical direction is wide at 70 degrees. Therefore, the ultrasonic sensor with an outside diameter D of 14 mm is less anisotropic, in the horizontal and vertical directions, than the ultrasonic sensor with an outside diameter D of 18 mm. It is to be noted that the half-decay angle used to evaluate the ultrasonic radiation and detection range is the angle between directions in which the ultrasonic radiation and detection sensitivity is less than 20 log 0.5 dB (approximately 60 dB) of the radiation and detection sensitivity at the front face (or at the zero degree direction).